Water dispersible polyisocyanates are commonly applied in the adhesives based on the aqueous resins as crosslinking agents. Particularly, the isocyanates are highly reactive to the compounds having active hydrogen groups so as to form, for example, urethane, urea, amide, allophanate, and biuret products. Thus, the polyisocyanates are advantageous to the molecular growth and crosslinking of the resins so as to extensively improve their adhesion, heat resistance, and mechanical performance when being used as adhesives. The characteristic of high reactivity of the polyisocyanates allows them to be an excellent crosslinking agent for the aqueous resins.
The preparations and applications of the water dispersible polyisocyanates are known and have been disclosed. For example, U.S. Pat. No. 4,663,377 discloses a polyisocyanate preparation dispersible in water and having an average NCO functionality of about 2.0 to 3.5, for use as an additive for aqueous adhesives based on polymers dispersed in water. According to the teachings of U.S. Pat. No. 4,663,377, polyether monols, such as poly ethoxylated ether monol exemplified in the examples provided thereof, are particularly suitable for the production of the water dispersible polyisocyanate preparation.
U.S. Pat. No. 5,200,489 relates to the use of a polyether monol to modify polyisocyanates based on 1,6-hexamethylene diisocyanate (HDI) or a mixture of HDI with up to 70% by weight, based on the weight of the diisocyanate, of another diisocyanate, to obtain a water dispersible polyisocyanate composition having an average NCO functionality of 2 to 6, and further provides a method for testing the water dispersibility of the polyisocyanate composition. The test results show that the polyether monol in an amount of about 10 wt % and having a molecular weight of 200 to 1000 g/mole is more favorable to improve the water dispersibility of the polyisocyanate composition.
U.S. Pat. No. 5,252,696 teaches a water dispersible polyisocyanate mixture obtained by reacting an aliphatic polyisocyanate having an NCO functionality of about 2.1 to 4.4, such as HDI trimer, with a polyether monol containing 5 to 9.9 ethylene oxide (EO) units, at an NCO/OH equivalent ratio of 4:1 to 120:1. The obtained water dispersible polyisocyanate mixture is suitable for two-component aqueous resins.
U.S. Pat. No. 5,563,207 is directed to a method for the preparation of a water dispersible polyisocyanate composition based on modified aliphatic and modified aromatic diisocyanates for use in aqueous coating compositions, especially an aqueous polyvinyl acetate coating, as a binder. Furthermore, the polyisocyanate composition of U.S. Pat. No. 5,563,207 is prepared by reacting a polyisocyanate component, e.g., modified HDI/toluene diisocyanate trimer, with a poly ethoxylated ether monol having a molecular weight of 550 g/mole and started with methanol.
U.S. Pat. No. 5,731,396 discloses a water dispersible polyisocyanate mixture that is prepared by reacting lacquer polyisocyanates, e.g., HDI trimer having an NCO functionality of 1.8 to 4.6, with hydrophilic polyether/ester monols obtained by reacting polyether monol with ε-caprolactone, at an NCO/OH equivalent ratio of 4:1 to 120:1. The prepared water dispersible polyisocyanate mixture is useful in the preparation of polyurethane (PU) plastics or as a crosslinking agent for water soluble or water dispersible lacquer binders.
U.S. Pat. No. 6,007,619 relates to a process for preparing mortar/concrete compositions by incorporating into the mortar/concrete compositions either before or during the mixture operation a water dispersible polyisocyanate. With reference to the examples provided in U.S. Pat. No. 6,007,619, a poly ethoxylated ether monol having an average molecular weight of about 350 g/mole or 550 g/mole and started with methanol is added to a HDI-based polyisocyanate, and then the mixture is heated to 100° C. for 3 hours, so as to form the water dispersible polyisocyanate.
U.S. Pat. No. 6,221,995 discloses a modified polyisocyanate which has a high emulsifying/dispersing ability in an aqueous resin composition and can be advantageously used as a curing or crosslinking agent. The modified polyisocyanate prepared by reacting an HDI-based isocyanurate group-containing polyisocyanate and a nonionic emulsifying agent, such as a polyoxyethylene nonylphenyl ether, has a hydrophile-lipophile balance (HLB) of 17 or less and provides preferred performance when used in aqueous coatings, inks, and adhesives.
In view of the known technologies, the water dispersible polyisocyanates are normally prepared by reacting an aliphatic polyisocyanate based on HDI trimer with a nonionic polyether monol having a low molecular weight. Although some prior references disclose that polyether polyols can be used in modifying the hydrophilic ability of the polyisocyanates, no concrete and detailed descriptions or even embodiments with actual preparation procedures and test data of the utility of polyether polyols are provided. Surprisingly, the inventors of the present invention have found that specific polyether diols, triols, a mixture of polyether diols and triols, or a mixture of polyether monols and polyether polyols can be selected to prepare a water dispersible polyisocyanate composition that is more favorable for use in aqueous resins as a crosslinking agent, compared with those prepared solely by polyether monols. Particularly, the aqueous resin adhesives crosslinked with the water dispersible polyisocyanate composition of the subject invention have increased mechanical properties, such as green strength, adhesive ability, including the initial and aging adhesion, and heat resistance.